Bathroom Exhaust Ducting Question

So I'm preparing to install a bathroom fan in a room thats never had one. The cheap fan I picked out has a 3" duct coming out of it. Because I have 6" insulted flexible ducting left over from another job, and I'd rather keep it big in case I have a change of heart down the road, what is the best way to go from 3" to 6"?

I found a couple reducers at the store, but the cost of the two I would need to pull this off is about $10 (almost as much as just buying a different size of ducting). Is there any alternative? I was thinking of maybe just clamping down real hard on the flexible duct until it meets, but I don't think that would hold very well in the long run...

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Bathroom Exhaust Ducting Question

The 3" duct tells me that the fan is undersized unless you bathroom is very small. In cold climates insualted duct is recommended because of condensation inside the duct, but rigid duct flows better. And where is that duct going to terminate?? Outside the attic I hope.